imagetools: support for creating attestations/signatures

Persist attestation manifest and any manifest cosign-based
signatures when creating new images.

When creating index from single-arch manifests where attestation
manifest is not inlined, it can be loaded from referrers API.
Note that for this to work the attestation manifest needs to be
in artifact type when image was built.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
(cherry picked from commit aab8171f824d13d703e869107e1720ad71d24bff)

build/build.go

@@ -723,7 +723,7 @@ func BuildWithResultHandler(ctx context.Context, nodes []builder.Node, opts map[
 								return err
 							}
 
-							dt, desc, _, err := itpull.Combine(ctx, srcs, indexAnnotations, false)
+							dt, desc, _, err := itpull.Combine(ctx, srcs, indexAnnotations, false, nil)
 							if err != nil {
 								return err
 							}